Conference Paper

Dynamic Event Trees without Success Criteria. Application to Full Spectrum LOCA sequences. Calculation of Damage Exceedance Frequency with Integrated Safety Assessment (ISA) Methodology

Conference: 38 Annual Meeting Nuclear Spanish Society

ABSTRACT The Integrated Safety Assessment (ISA) methodology, developed by the Spanish Nuclear Safety Council (CSN), has been applied to obtain the Dynamic Event Trees (DETs) for Full Spectrum Loss of Coolant Accidents (LOCAs) of a Westinghouse 3-loop PWR plant. The purpose of this ISA application is to obtain the Damage Exceedance Frequency (DEF) for the LOCA Event Tree by taking into account the uncertainties in the break area and the operator actuation time needed to cool down and depressurize reactor coolant system by means of steam generators. Simulations are performed with SCAIS, a software tool which includes a dynamic coupling with MAAP thermal hydraulic code. The results show the capability of the ISA methodology to obtain the DEF taking into account the time uncertainty in human actions.

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    ABSTRACT: Over the past years, many Nuclear Power Plant organizations have performed Probabilistic Safety Assessments to identify and understand key plant vulnerabilities. As part of enhancing the PSA quality, the Human Reliability Analysis is essential to make a realistic evaluation of safety and about the potential facility's weaknesses. Moreover, it has to be noted that HRA continues to be a large source of uncertainty in the PSAs. Within their current joint collaborative activities, Indizen, Universidad Politécnica de Madrid and Consejo de Seguridad Nuclear have developed the so-called SIMulator of PROCedures (SIMPROC), a tool aiming at simulate events related with human actions and able to interact with a plant simulation model. The tool helps the analyst to quantify the importance of human actions in the final plant state. Among others, the main goal of SIMPROC is to check the Emergency Operating Procedures being used by operating crew in order to lead the plant to a safe shutdown plant state. Currently SIMPROC is coupled with the SCAIS software package (Izquierdo et al., 2008), but the tool is flexible enough to be linked to other plant simulation codes.SIMPROC–SCAIS applications are shown in the present article to illustrate the tool performance. The applications were developed in the framework of the Nuclear Energy Agency project on Safety Margin Assessment and Applications (SM2A).First an introductory example was performed to obtain the damage domain boundary of a selected sequence from a SBLOCA. Secondly, the damage domain area of a selected sequence from a loss of Component Cooling Water with a subsequent seal LOCA was calculated. SIMPROC simulates the corresponding human actions in both cases.The results achieved shown how the system can be adapted to a wide range of purposes such as Dynamic Event Tree delineation, Emergency Operating Procedures and damage domain search.
    Nuclear Engineering and Design. 01/2011;
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    ABSTRACT: Binary decision diagrams (BDDs) are a well-known alternative to the minimal cutsets (MCS) approach to assess Boolean reliability models. While the application of fault tree analysis can be considered to be consolidated, its application to the event trees involved in the probabilistic safety assessment (PSA) studies of the nuclear industry require extended efforts. For many real PSA models the full conversion procedure remains out of reach in terms of computational resources owing to their size, non-coherency, redundancy, and complexity. A potential solution to improve the quality of assessment methods is to design hybrid algorithms that combine the information derived from the calculation of MCS with the BDD methodology.
    Proceedings of the Institution of Mechanical Engineers Part O Journal of Risk and Reliability 01/2009; 223(4):301-311. · 0.62 Impact Factor
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    ABSTRACT: The Integrated Safety Assessment (ISA) methodology, developed by the Consejo de Seguridad Nuclear (CSN), Spanish Nuclear Regulatory Body, has been applied to a thermo-hydraulic analysis of Zion NPP for sequences with loss of the Component Cooling Water System (CCWS). The ISA methodology allows obtaining the damage domain (the region where the PCT limit is exceeded) for each sequence of the dynamic event tree as a function of the operator actuations times (secondary side cooling and recovery of CCWS) and the time of occurrence of stochastic phenomena (seal LOCA), and computing from it the exceedance frequency by integrating the dynamic reliability equations proposed by ISA 12 . For every sequence, several data are necessary in order to obtain its contribution to the global exceedance damage frequency. These data consist of the results of the simulations performed with MAAP and TRACE codes that are inside of the damage domain and the time-density probability distributions of the manual actions and the time of seal LOCA occurrence. Reported results show an slight increment of the exceedance damage frequency for this kind of sequences in a power uprate from 100% to 110%.
    OECD/CSNI Workshop on Best Estimate Methods and Uncertainty Evaluations; 11/2011

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